Purified microvascular endothelial (MVE) cell preparation from rat and guinea pig fat pad bound diamine oxidase (DAO) in aqueous extracts of rat and human placenta, and in purified preparations of DAO from rat placenta. Scatchard plots indicated the presence of high and low affinity binding sites for DAO. MVE cells also bound purified lipoprotein lipase (LPL). However, bound LPL was not displaced by DAO activity except at high concentrations of DAO. Conversely, bound DAO was only partially displaced by high concentrations of LPL. Binding of both enzymes was blocked by heparin. Each enzyme may thus be bound to two or more populations of sites and one population may bind both enzymes.